In restaurant kitchens fume hoods are necessary to remove cooking odors, especially over deep fat frying equipment where the fumes include a considerable amount of grease.
Prior art systems include an exhaust fan and a duct system for drawing the air from the cooking surface into the duct and out an outlet. They sometimes include screens or filters for collecting the grease. Since such screens require considerable cleaning, the preferred system has been to let the grease be eliminated from the air by having it stick to the inside surface of the duct carrying the air. This not only is an inefficient and unsatisfactory method of removing the grease, but also, because of the heat involved, the grease tends to bake onto the inner surface of the ducts and is difficult to remove. This also creates a fire possibility. In fact, most fire codes require a spring-loaded fire damper to close the top of the duct in the event of fire.
In systems such as these, inside of the duct is cleaned when the hood is not being operated by using one or more spray nozzles permanently mounted inside the duct to scrub down the baked on grease.
A variation on this prior art technique is a system in which one or possibly more spray nozzles are mounted somewhere within the duct. These operates while the hood is in operation and have as their purpose to cool the air so that better condensation of the grease on the sides of the duct will be obtained. Normally, this provides only nominal air-water contact so only a small amount of grease will actually be removed by the water. To the extent that grease is removed by the water, it could cause clogging unless there is a periodic detergent cycle in which the fan is turned off; otherwise the grease may be carried in the water droplets and stick to the fan blade that operates the unit.
"Waterfalls" have been used in industrial air scrubbers, in association with other steps, but normal waterfalls are subject to being "punctured" by an air stream, i.e., develops holes or voids in the waterfall permitting such of the air to pass through the holes without contacting the water. See, for example, the patents to Fisher U.S. Pat. Nos. 2,259,032 and 2,354,674, McIlvaine U.S. Pat. No. 3,077,714, and Wisting U.S. Pat. Nos. 3,336,733, 3,406,498 and 3,448,562.